Wide-band differential amplifier using gm-cancellation

ABSTRACT

A wide-band differential amplifier includes four MOS transistors connected in a source-follower configuration for gm-cancellation. Accurate gain and low distortion of the amplified signals results over the frequency band.

BACKGROUND OF THE INVENTION

This invention relates generally to differential amplifiers, and moreparticularly the invention relates to a differential amplifier havingaccurate gain over a wide bandwidth with low signal distortion.

Conventional wide-band MOS transistors and amplifiers are discussed byJohn M. Steininger in "Understanding Wide-Band MOS Transistors," IEEECircuits and Systems, May 1990, pp. 26-31. Conventional MOS differentialamplifiers have current sources which introduce current imbalance whichleads to distortion by variations of the operating conditions.

SUMMARY OF THE INVENTION

The present invention is an MOS wide-band differential amplifier havingaccurate gain with low signal distortion.

A feature of the invention is a source-follower configuration of fourinput transistors which achieves gm-cancellation and fully cancels outany distortion introduced by variation of operating conditions.

The invention and objects and features thereof will be more fullyunderstood from the following description and appended claims when takenwith the drawing.

BRIEF DESCRIPTION OF THE DRAWING

The drawing is a schematic of a differential amplifier in accordancewith the preferred embodiment of the invention.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

Referring now to the drawing, the amplifier 10 in accordance with theinvention has four MOS transistors, 12, 14, 16, and 18, each of whichhas a source (20, 22, 24, and 26, respectively), a drain (28, 30, 32,and 34), respectively, and a gate (36, 38, 40, and 42), respectively.The first and third transistors 12 and 16 are serially connected in asource follower configuration with a first current source 46 and a firstcurrent mirror 41 which includes a fifth MOS transistor 60, alsoincluding a source 62, drain 64, and gate 66. Likewise, the second andfourth transistors 14 and 18 are serially connected in a source followerconfiguration with a second current source 48 and a second currentmirror 44 which includes a sixth MOS transistor 68 having a source 70,drain 72, and gate 74. The differential inputs to the amplifier, -V_(in)and +V_(in) are applied to the gates of the first and second transistors12 and 14, respectively, while the gates of the third and fourthtransistors 16 and 18, respectively, are cross-coupled to the sources ofthe transistors 14 and 12. A first resistor Rd1 (also identified aselement 58) is coupled between the sources of the third and fourthtransistors 16 and 18.

The amplifier further includes a third current source 76, which isserially connected with a seventh MOS transistor 78, also having asource 80, drain 82, and gate 84, and which forms a part of the firstcurrent mirror 41. Also, the first current mirror 41 includes aconnection between the gates 66 and 84 of the fifth and seventhtransistors 60 and 78 to effect mirroring the currents in the respectivetransistors. Similarly, a fourth current source 88 is serially coupledwith an eighth MOS transistor 90, also having a source 92, drain 94, andgate 96 and which forms a part of the second current mirror 44. Thecurrent mirror also includes a lead 98 between the bases 74 and 96 ofthe sixth and eighth transistors 68 and 90, respectively.

Finally, a second resistor Rd2 (100) is connected between the drains 82and 94 of the seventh and eighth transistors 78 and 90, the outputs ofthe amplifier then being taken across the resistor Rd2 (100) at thedrains 82 and 94 of the seventh and eighth transistors 78 and 90. Thethird and fourth current sources 76 and 88 thus generate currents whichare a function of Vcm taken from the second resistor 100.

The configuration in this amplifier provides total cancellation of gmwhich results in a voltage drop across Rd1 (58) which is identical tothe differential signal at the amplifiers inputs. Hence, thenonlinearity of the transistors (especially when applied to largesignals) is fully canceled. The AC current through Rd1 (58) is thenamplified by means of the programmable current mirror ratio and isdumped into the second resistor Rd2 (100) at the output stage. The gainof the circuit is (Rd2/Rd1) A, with the value A being the current mirrorratio.

While the invention has been described with reference to a specificembodiment, the description is illustrative of the invention and is notto be construed as limiting the invention. Various modifications andapplications may occur to those skilled in the art without departingfrom the true spirit and scope of the invention as defined by theappended claims.

I claim:
 1. A wide-band differential amplifier comprising:first, second,third and fourth MOS transistors (12, 14, 16, and 18) each having asource, a drain, and a gate, a first current mirror having a fifth MOStransistor having a source, drain, and gate connected to the drain ofsaid first transistor, a second current mirror having a sixth MOStransistor having a source, drain, and gate connected to the drain ofsaid second transistor, a first current source serially connected withsaid third transistor, said first transistor and said first currentmirror, a second current source serially connected with said fourthtransistor, said second transistor, and said second current mirror,means connecting said gate of said third transistor to said source ofsaid second transistor, and means connecting said gate of said fourthtransistor to said source of said first transistor, means for applying afirst input (-V_(in)) to said gate of said first transistor and meansfor applying a second input (+V_(in)) to said gate of said secondtransistor, a first resistor Rd1, connected between said source of saidthird transistor and said source of said fourth transistor, a thirdcurrent source serially connected with a seventh MOS transistor having asource, drain, and gate, and which forms a part of said first currentmirror, means connecting said gates of said fifth and seventhtransistors of said first current mirror, a fourth current sourceserially connected with an eighth MOS transistor having a source, drain,and gate, and which forms a part of said second current mirror, meansconnecting said gates of said sixth and eighth transistors of saidsecond current mirror, and a second resistor, Rd2, connected betweensaid drains of said seventh and eighth transistors, the outputs of saidamplifier being taken at said drains of said seventh and eighthtransistors.